This invention relates to a fiber-reinforced plastics leaf spring (hereinafter referred to as "an FRP leaf spring") which is constructed by embedding a plurality of continuous fibers bearing prescribed different thicknesses or diameters in a plastics material in a predetermined distribution.
The plastics material of the conventional leaf spring of the above-mentioned type is chiefly prepared from thermosetting resin. The continuous fiber (simply referred to as "a fiber") of said leaf spring is mainly formed of a glass fiber having a diameter of 10 to 25 microns. The conventional leaf spring comprises a large number of glass fibers which continuously extend in a prescribed length along the prescribed lines defined in the leaf spring. Such an FRP leaf spring has come to be widely accepted, for instance, for the suspension in rolling stock. Consequently, there has been a strong demand to solve problems related to improvements in durability, particularly at elevated temperatures. A decline in the high temperature durability of the conventional FRP leaf spring results from the fact that when said leaf spring is subjected to a temperature of 40.degree. to 100.degree. C. or higher, the fibers contained in the leaf spring tend to buckle on that side of the leaf spring on which compression stress occurs.
The present inventors have conducted studies for resolution of the aforementioned drawbacks, and experimentally discovered that finer fibers contained in the FRP leaf spring assure higher durability at room temperature, while fibers of larger diameter used on that side of the FRP leaf spring which undergoes compression improves the durability of said leaf spring at a high temperature. Brief description will now be given of the experimental data. The inventors provided three sample leaf springs which respectively comprised continuous fibers measuring 13, 17 and 23.5 microns in diameter which equally measure 7 mm in thickness, 20 mm in width and 300 mm in length. The fibers are distributed through the leaf spring substantially in the same manner. The samples were subjected to a durability test substantially similar to process A specified in ASTM 671-63T, obtaining the results below (number of cycles) from the repeated load test. Results of the tests are as follows.
______________________________________ Result Diameter of fiber ______________________________________ 71.about.100 .times. 10.sup.4 13 .mu.m 37.about.85 .times. 10.sup.4 17 .mu.m 44.about.79 .times. 10.sup.4 23.5 .mu.m ______________________________________
The above experiment showed that the leaf spring including finer fibers proved more durable. Similar tests to those described above were carried out on another sample leaf spring which had the same measurements as described above and was constructed by embedding fibers of 13 micron diameter from the surface on which a tensile stress occurred to a depth of about 2 mm from said surface, and embedding fiber of 17 micron diameter in the other regions of the leaf spring. From the above-mentioned experiment, a result of (65 to 90).times.10.sup.4 is obtained. On the other hand, the first mentioned three sample leaf springs were subjected to a high temperature test at 60.degree. C., giving the following results. A sample leaf spring comprising fiber of 13 micron diameter withstood (40 to 58).times.10.sup.4 load tests. A sample leaf spring composed of two kinds of fibers having diameters of 13 and 17 microns withstood (51 to 82).times.10.sup.4 load tests. The latter experiment proved that when a compression stress was applied at high temperature, larger diameter fibers embedded in that region of the leaf spring in which a compression stress was applied ensured the higher durability of the leaf spring. The above-mentioned experimental results are derived from the fact that the resin softens at high temperature and grips fibers with a weaker force, tending to give rise to the buckling of the fibers, but that the application of larger diameter fibers on that side of the leaf spring on which a compression stress is applied increases the force resisting the buckling of the fibers.
Consideration should be given to the following points in manufacturing a FRP leaf spring. Finer fibers should be applied in a larger number than fibers of greater diameter, in order to ensure a sufficiently large volume. For example, where a leaf spring is manufactured by employing reinforcing fibers having a larger diameter, the number of fibers required is smaller than the number of fibers required where the same leaf spring is manufactured by employing finer reinforcing fibers. Therefore, the manufacturing efficiency becomes high when the larger-diameter fibers are employed. However, provision of finer fibers is accompanied with difficulties in bundling, twisting or properly arranging individual fibers, and since the interfiber space is narrow, it is difficult to impregnate resin fully to the corners of said interfiber space. This means that the continuous manufacture of a FRP hoop for making the leaf spring from finer fibers, is unavoidably carried out at a lower speed than when producing the FRP leaf spring from fibers of greater diameter fibers.